The improvement of aircraft gas turbines and electrical energy generation by means of static gas turbines is moving ever more into the foreground owing to the high efficiencies achievable with gas turbines. In a gas turbine, air taken in is compressed and supplied to a combustion chamber. In the combustion chamber, a mixture of the supplied air and a fuel is ignited and the hot combustion gases, which are at a high pressure, are delivered to a turbine section of the gas turbine. The combustion gases are used as a working medium which causes the turbine to rotate, typically with a frequency of from 50 to 60 Hz. The working medium arrives at the first turbine blades with a temperature of around 1200° C. Owing to the rotation, the hot turbine blades are exposed to high static loads due to centrifugal forces. Furthermore, dynamic loads are induced by the hot gas flowing in. Depending on the fuel being used, more or less strong corrosive loads also occur on the components of the turbine section, in particular on the turbine blades.
For example, the rotor blades of the first rotor blade row of the turbine are therefore often made from so-called refractory superalloys based on nickel, cobalt or iron. Such superalloys are known for example from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949.
Owing to the usually extremely complex blade design and the relatively low suitability of such alloys for being shaped, turbine blades are produced by the vacuum casting method. The solidification process is controlled so as to form a directional microstructure in the component, which has a particularly advantageous orientation in relation to the loads subsequently occurring. The term “directional microstructure” is intended to mean both monocrystalline structures and structures having a grain structure in which the extent of the grains has a common preferential direction. In the latter case, the grains may for example have a larger dimension in a particular preferential direction than in the other directions (so-called columnar crystals). Components having such a grain structure are also referred to as directionally solidified components.
The effect of the constant high static and dynamic loads on turbine components, and in particular rotor blades of the turbine section, is that they are subjected to revision at regular intervals in order to assess them in respect of their suitability for further use. In the scope of this revision process, damaged components are either replaced or repaired. Besides great material losses due to corrosion, fatigue cracks in particular are the most frequently occurring reasons for replacement of the components. Particularly in the case of rotor blades, such fatigue cracks occur transversely to the blade longitudinal axis.
Owing to the high material and production costs for turbine components having a directional microstructure, a power plant operator or a manufacturer of aircraft turbines must take into account in relatively high costs, for example to replace turbine blades, for the maintenance of a gas turbine system. Against the background of a rising cost pressure, repair methods which restore the operability of a damaged turbine component, in particular a damaged rotor blade, therefore become increasingly more attractive. A repaired component may then be reinstalled in the gas turbine system and may be used further until the next revision process.
One possibility for repairing damaged components is soldering. For example, a solder material is introduced into a crack in the component and is bonded to the material of the superalloy by the action of heat. The soldering process may be carried out isothermally, i.e. at constant temperature, or with the use of a temperature gradient.
A method for the isothermal soldering of monocrystalline objects is described for example in EP 1 258 545. In this case, a crack is filled with a solder material that resembles the superalloy of the component in its composition, and which is then kept at a temperature above the melting point of the solder material for a prolonged period of time. Boron is added to the solder material in order to lower the melting temperature. Owing to the high temperature, diffusion processes take place that induce concentration equilibration between the solder material and the superalloy, which leads to solidification of the solder material. With gap widths up to about 200 μm, the solidified solder material adopts the directional microstructure of the surrounding superalloy. Boron, however, may lead to the formation of brittle borides which impair the properties of the component in the region of the repaired site. Furthermore, the method is not suitable for gap widths of more than about 200 μm.
A method for soldering directionally solidified or monocrystalline components with the use of a temperature gradient is described for example in EP 0 870 566. In this method a solder alloy that consists for example of the basic material of the part to be repaired, with the addition of one or more elements which lower the melting point, is applied onto the site to be repaired. The part to be repaired is moved with the applied solder alloy through a heated zone, the temperature applied in this heated zone by the heating being above the melting temperature of the solder alloy but lower than the local pre-melting temperature of the part. Partial melting of the superalloy of the part to be repaired does not therefore take place. A thermal gradient is generated in the solder product owing to the movement, which leads to directional solidification of the solder product. Constituents that lower the melting point are also added to the solder product in this case, which for example in the case of boron may negatively affect the properties of the repaired site.